| Background and objectivesThe cortex of human brain seems to play exclusive role in high cognitive functions. However, patients with subcortical structures lesions appeared to be impairment in high cognitive functions, suggesting that subcortical structures may play an important role in some high cognitive functions. The subcortical structures are mainly consisted of the basal ganglia and other subcortical gray matter. Studies on the roles of the subcortical structures were always an interesting and hot topic. The human striatum, as main parts of the basal ganglia, developed to be a subcortical central region on motorial effects due to the high development of the cerebral cortex. However, there is growing evidence suggesting that the subcortical structures, such as striatum, are also related to the high cognitive function in human brain. A newly discovered learning and memory area in the brain, termed marginal division (MrD) of the neostriatum, had been confirmed to be associated with the declarative and digital working memory. As one of the important high cognitive function, learning and memory had always been foci in neuroscience research area. Researches on the neural basis of memory mainly focused on cerebral cortex; comparatively there wss only limited studies investigating the roles of these regions in memory. The majority of these limited studies generally focused on non-declarative memory. Baddeley first introduced the concept of working memory based on short-term memory. A large number of studies indicated that the cerebral cortex, especially the prefrontal lobe, played an important role in manipulating the information of working memory. The neural basis of declarative memory had been examined to rely on medial temporal system for a long time. The striatum seemed to be related only to non-declarative memory rather than declarative memory, and little was known about the role of subcortical structures associated with working memory. Semantic memory, an important declarative memory, refers to our general knowledge of concepts and facts, which are quite different from the specific memory of personal experiences. Nearly all studies of semantic memory were focused on the semantic retrieval phase of the brain but not on the encoding phase of the brain, and observations of semantic memory were described in relation to the functional specialization but not to the functional integration of the brain. Spatial information refers to the external information of the objects including the site, the direction, the distance, and so on. Nearly all reports of spatial memory suggested that the cerebral cortex were responsible for processing the information of spatial memory, and the role of the different cortical regions contributed to spatial memory is still a matter of debate. However, the subcortical structures diseases, such as Parkinson's disease and Huntington's disease, may develop the visual spatial disorder during the early stage of disease, suggesting the subcortical structures probably subserve to spatial memory. Learning and memory is complicated high cognitive function; most techniques of research on it were invasive. It was always very difficult to examine the working process in vivo brain by invasive techniques. With the development of modern science and researches, neuroimaging techniques, especially functional neuroimaging techniques, have been applied extensively in research and clinic of the neuroscience area. Functional magnetic resonance imaging (fMRI) is one of the non-invasive functional neuroimaging techniques, which has been applied in neuroscience research area since its invention in the early 1990s. Its basic principles are related to the physiology of the blood oxygenation level-dependent (BOLD) contrast mechanism and to the acquisition of functional time-series with echo planar imaging (EPI). fMRI has rapidly assumed a leading role among the techniques used to localize brain activity. The spatial and temporal resolution provided by state-of-the-art MR technology and its non-invasive character, which allows multiple studies of the same subject, are some of the main advantages of fMRI over the other functional neuroimaging modalities that are based on changes in blood flow and cortical metabolism. fMRI can offer a good prospect to investigate the fundamental mechanism of learning and memory.The present study attempted to investigate the collaborative activities of the cortical and subcortical structures during the semantic and spatial working memory processing by fMRI study, to examine the common neural network in the brain underlying two different semantic working memory tasks (the semantic category working memory task and the paired-word associative learning and memory task), to compare the neural basis for different phases of semantic working memory, to make sure about the role of different brain regions involved in spatial working memory processing. Study on the role of the subcortical structures in human high cognitive function helps us further understanding of fundamental mechanism related to cognitive impairment and dementia, specifically concerned with the dementia in basal ganglia disorders such as Parkinson's disease or Huntington's disease. Materials and MethodsWe selected semantic category, paired word, and spatial site as study materials. Twelve and sixteen right-handed healthy volunteers (20-23 years old) were recruited to participate respectively in the semantic category working memory task and the paired-word associated learning and memory task, while ten right-handed healthy volunteers (20-23 years old) participated the spatial working memory task. Functional imaging was carried out by a 1.5 Tesla Magnetom scanner while healthy volunteers performed the memory tasks. A control task was performed for the block-design in each test. Memory task and baseline were arranged alternatively and each task included four cycles. SPM99 (Welcome Department of Cognitive Neurology; http://www.fil.ion.ucl.ac.uk/spm) was used to analyze the fMRI experimental data and to identify the activated brain regions. Images from the first 6 seconds of acquisition of each session were removed from further functional data processing to minimize the transit effects of hemodynamic responses. The preprocessing of fMRI data, including the slice time correction, the realignment, the coregistration, the normalization and the smoothing, were preformed. Activation maps were generated by using a cross-correlation method, where the activity of each pixel was correlated to a boxcar function that was convolved with the canonical hemodynamic response function. Subject-specific linear contrasts, including the encoding condition versus baseline and the retrieval condition versus baseline for each of the effects of interest, were assessed. These contrasts were entered into a standard SPM second-level analysis treating subjects as a random effect, using a one-sample t-test. The expected mean difference value for the t-test was set to zero. A voxelwise intensity threshold (P≤0.005 or P≤0.001) and a spatial extent (10 or 5 voxels as the minimum cluster size) were set for multiple comparisons. All coordinates reported were in Talairach space.Results From the above design of experiments, the results were showed as the following: 1. Brain regions of activation in both cerebral cortex and subcortical structures were observed in the encoding and retrieval phases of the semantic category working memory task. During the encoding phase of this memory task, the cerebral cortex including the middle gyrus (BA46/9/6), the superior gyrus (BA6) and the inferior gyrus (BA47) of the frontal lobe, the superior parietal lobule (BA7) and the supramarginal gyrus (BA40) of the parietal lobe, the fusiform gyrus (BA37) of the temporal lobe, the insula (BA13) and the occipital lobe (BA19) were activated with left hemisphere predominance. While the subcortial structures including caudate, claustrum, and thalamus were activated. Other brain regions such as cerebellum were also activated. The most striking activation region was the left middle frontal gyrus (BA46/9/6). During the retrieval phase of this memory task, the cerebral cortex including the frontal lobe (BA46/9/6/47), the cingluate gyrus (BA32), the parietal lobe (BA7/40/39), the occipital lobe (BA17/18/19) and the insula (BA13) were activated with left hemisphere predominance. While the subcortical structures including striatum and its MrD, claustrum, and thalamus were activated. The activations of the cerebellum and some sub-gyral white matter were also observed. The most striking activation region was the left superior frontal gyrus (BA6). 2. Brain regions of activation in both cerebral cortex and subcortical structures were observed in the encoding and retrieval phases of the paired-word associative learning and memory task. During the encoding phase of this memory task, the cerebral cortex including the occipital lobe (BA18/19), the frontal lobe (BA46/9/6/47), the parietal lobe (BA7/40/39), and the temporal lobe (BA20/21/37) were activated with left hemisphere predominance. While the subcortical structures including striatum and its MrD, and thalamus were activated. The activations of the cerebellum and some sub-gyral white matter were also observed. The most striking activation region was the left the occipital lobe (BA18/19). During the retrieval phase of this memory task, the cerebral cortex including the frontal gyrus (BA46/9/6/8/10/47), the parietal lobe (BA7/40) and the occipital lobe (BA18/19) were activated with left hemisphere predominance. While the subcortical structures including striatum and its MrD, the substantia nigra and the red nucleus were activated. The activations of the cerebellum and some sub-gyral white matter were also observed. The most striking activation region was the left parietal lobe (BA7). 3. The activated brain areas displayed some similar alterations from encoding phase to retrieval one of the two memory tasks. The stronger and major foci activated areas in cerebral cortex from the left ventrolateral and mid-dorsolateral regions during encoding phase transferred to the left dorsolateral regions during retrieval phase. The intensity and extent of activated regions in the cerebral cortex during encoding phase were stronger than those during retrieval phase, whereas the opposite activation pattern was found for the subcortical structures during the two phases. 4. The major significant activated brain regions for both memory tasks were overlapped, including the middle and superior frontal gyrus(BA6/9/46), the inferior frontal gyrus(BA47), the superior parietal lobule(BA7), the supramarginal gyrus(BA40) and the occipitotemporal(BA37/19) region, the striatum, the thalamus and the cerebellum. The cerebral cortex was activated with a strong left lateralization during the two tasks, and the common activated areas of the two different semantic working memory tasks in the cerebral cortex formed an arcuate area surrounding the perisylvian language cortex zone.5. Both cortex and subcortical structures were activated in the spatial working memory task. The brain cortex areas including the bilateral precuneus lobules and superior lobules as well as supramarginal gyrus of the parietal lobe (BA7/40), the bilateral superior and middle as well inferior gyrus of the prefrontal lobe (BA6/9/47), the bilateral occipitotemporal lobes (BA19/37), the right parahippocampal gyrus (BA30) and the right cingulated gyrus (BA25) were activated during the task. The subcortical structures including the right caudate of the neostriatum, the right claustrum, the left thalamus and the left substantia nigra in midbrain were activated during the task. Meanwhile, the bilateral cerebellums were also prominently activated. The most striking activation region was the parietal lobe (BA7/40). The differences of the intensity between the left parietal lobe and the bilateral frontal lobes, the right parietal lobe and the left frontal lobe were significant (P<0.05) .ConclusionsFrom the above results of this study, we can draw the conclusion as the following: our study demonstrate that the information of the semantic working memory in human brain is manipulated by the collaborative activities within cortex, and between cerebral cortex and subcortical structures, and the subcortical structures are involved in declarative memory and working memory. MrD is further confirmed to be a new area associated with learning and memory by this study because it is activated during the semantic working memory tasks. The encoding and retrieval phases of the semantic working memory depend on different neural foundations in human brain. The executions of the semantic memory mostly rely on the left ventrolateral and mid-dorsolateral cortical regions during the encoding phase, whereas it primarily depend on the left dorsolateral cortical regions and the subcortical structures during the retrieval phase. Subcortical structures may play an important role in the retrieval phase of the semantic working memory task, and also is a key component of the semantic working memory neural circuit. The results indicate that there is a general neural network contributing to the semantic working memory. The network include the left superior, middle, inferior frontal gyri (BA46/9/6/47), left superior parietal lobule (BA7), left supramarginal gyrus (BA40), left occipitotemporal region (BA19/37), striatum, thalamus and cerebellum. The common activated cortex areas form an arc surrounding the perisylvian language cortex zone with strong left lateralization. There is a neural circuit of semantic working memory as the following: the ventrolateral and mid-dorsolateral cortical regions receive the impulses, analyze and encode the information; the subcortical structures, mainly the basal ganglia, deliver the information to the dorsolateral cortical regions to accomplish semantic information retrieval. The subcortical structures as well as the cerebral cortex contribute to the spatial working memory, and the human parietal lobes are crucial brain regions in manipulating information of the spatial working memory rather than the frontal lobes.
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